Process for fabricating thin film transistors

ABSTRACT

Transistors are formed by depositing at least one layer of semiconductor material on a substrate comprising a polyphenylene polyimide. The substrate permits the use of processing temperatures in excess of 300° C. during the processes used to form the transistors, thus allowing the formation of high quality silicon semiconductor layers. The substrate also has a low coefficient of thermal expansion, which closely matches that of silicon, thus reducing any tendency for a silicon layer to crack or delaminate.

REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 09/836,884,filed Apr. 17, 2001 (now U.S. Pat. No. 6,825,068), which claims priorityof Provisional Application Ser. No. 60/197,731, filed Apr. 18, 2000.

BACKGROUND OF THE INVENTION

This invention relates to a process for fabricating thin filmtransistors.

Thin film transistors (TFT's) are known to be useful for, inter alia,controlling various types of display; for example TFT's are commonlyused to control liquid crystal displays used in portable computers andsimilar electronic devices. TFT's can also be used to controlelectrophoretic displays; see, for example, Applications Ser. Nos.60/132,642, filed May 5, 1999 and Ser. No. 09/565,413, filed May 5, 2000(now U.S. Pat. No. 7,030,412); Applications Ser. Nos. Ser. No.60/144,943, filed Jul. 21, 1999, 60/147,989, filed Aug. 10, 1999, andSer. No. 09/621,000, filed Jul. 22, 2000 (now U.S. Pat. No. 6,842,657);Applications Ser. Nos. 60/151,547, filed Aug. 31, 1999, and 09/652,486,filed Aug. 31, 2000 (now U.S. Pat. No. 6,312,971); Applications Ser.Nos. 60/151,715 and 60/151,716, both filed Aug. 31, 1999, and09/651,710, filed Aug. 31 2000 (now U.S. Pat. No. 6,498,114); andApplications Ser. Nos. 60/151,549, filed Aug. 31, 1999 and Ser. No.09/650,620, filed Aug. 30, 2000 (now U.S. Pat. No. 6,545,291). Thedisclosures of all these copending applications are herein incorporatedby reference. See also the corresponding International ApplicationsPublication Nos. WO 00/67327; WO 01/08241;WO 01/17029; WO 01/17040; andWO 01/17041.

Although most TFT's have hitherto been fabricated on rigid substrates,there is increasing interest in fabricating TFT's on flexiblesubstrates, especially flexible polymeric films. TFT's fabricated onsuch flexible substrates could form the basis for large displays whichwould be light-weight yet rugged, thus permitting their use in mobiledevices. TFT's based upon amorphous silicon semiconductors areattractive for use on such flexible substrates since they allowfabrication with a minimum number of process steps and with a lowthermal budget. Amorphous silicon transistors have been fabricated onultra-thin stainless steel substrates (see, for example, Ma et al.,Applied Physics Letters, 74(18), 2661 (1999)) and on polyimide films(see Gleskova et al., IEEE Electron Device Letters, 20(9), 473 (1999)).

However, the polyimide used in the process described in the latterpaper, sold commercially under the name “Kapton” (Registered Trade Mark)has a glass transition temperature of only about 300° C., whichrestricts the temperatures which can be employed during the fabricationprocess, and results in a less satisfactory amorphous siliconsemiconductor layer. This polyimide also has a high moisture absorption(about 4 percent by weight) and such high moisture absorption can resultin swelling of the substrate and consequent cracking of thin layersdeposited on the substrate, or delamination of thin layers from thesubstrate. Although stainless steel substrates can withstand processtemperatures much higher than 300° C., such substrates require bothpassivation and planarization steps before transistors can be fabricatedthereon. Passivation is required to ensure proper electrical isolationbetween adjoining metal conductors to be formed on the substrate, and toensure that potential contaminants within the stainless steel do notdiffuse into the transistors. Stainless steel substrates do, however,have the advantages of high dimensional stability and ease of handlingin a manufacturing environment.

It has now been discovered that certain types of polyimides possessproperties which render them very suitable for use as substrates in thefabrication of TFT's. These polyimide substrates may be used with orwithout a metal backing layer.

SUMMARY OF THE INVENTION

Accordingly, this invention provides a process for forming at least onetransistor on a substrate by depositing on the substrate at least onelayer of semiconductor material. In the present process, the substratecomprises a polyphenylene polyimide. This process is especially intendedfor the formation of amorphous silicon transistors, and in such a casethe semiconductor material is of course an amorphous silicon.

This invention also provides a transistor formed on a substratecomprising a polyphenylene polyimide, the substrate bearing at least onetransistor.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 of the accompanying drawings is a schematic cross-section througha single transistor of a thin film transistor array formed on apolyimide substrate by the process of the present invention.

FIG. 2 shows a cross-sectional view of an electrophoretic display of thepresent invention incorporating transistors as shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Preferred polyphenylene polyimides for use in the present process arethose sold commercially under the trade names Upilex-S and Upilex-VT byUBE America, Inc., 55 East 59th Street, 18th Floor, New York N.Y. 10022.Both these materials are stated by the manufacturer to be of theformula:

in which R is an alkylene group. These polyimides are derivatives ofbiphenyl-3,3′,4,4′-tetracarboxylic acid and an α,ω-alkanediamine, forexample 1,6-hexamethylene diamine (formally hexane-1,6-diamine). Themain difference between the two materials is that Upilex-S is a simplepolyimide film, while Upilex-VT is surface-treated on one face tofacilitate hot lamination of the film, without adhesive, to ceramics ormetal foils. For purposes of the present invention, the Upilex VT may belaminated to a stainless steel backing film. Another preferred polyimidefor use in the present process is Upilex-50SS, from the samemanufacturer.

Polyphenylene polyimides have glass transition temperatures considerablyhigher, and water absorptions considerably lower, than those of theother polyimides previously used as transistor substrates. Thecommercial Upilex materials already mentioned have glass transitiontemperatures in excess of 400° C. and water absorptions not greater thanabout 1.4 percent. These high glass transition temperatures allow theuse of higher temperatures (greater than about 300° C., and preferablygreater than about 400° C.) in the fabrication process than has hithertobeen possible with the polyimides previously used as transistorsubstrates, and these higher process temperatures result in higherquality silicon layers having higher mobility and low off-state currentleakage. Polyphenylene polyimides also have the advantages of highdimensional stability during processing and smooth surfaces, which isimportant for the deposition of the thin layers of material used in theformation of thin film transistors; for example, the aforementionedUpilex-50SS has a dimensional stability of about 0.01% and an averagesurface roughness of about 20 to 30 nm. An additional advantage ofpolyphenylene polyimides is their low coefficients of thermal expansion,which are typically about 2-10×10⁻⁶° C.⁻¹, in contrast to the 35×10⁻⁶°C.⁻¹ typical of the polyimides previously used. Since silicon has acoefficient of thermal expansion of about 3×10⁻⁶° C.⁻¹, the coefficientof a polyphenylene polyimide substrate is much more closely matched to asilicon layer deposited thereon, so that the silicon layer is much lessprone to cracking and/or delamination.

The aforementioned properties of polyphenylene polyimide substratesrender the present process well adapted for use in a roll-to-rollprocess, in which deposition of the semiconductor material is effectedon a continuous web of the polyimide substrate.

As already mentioned, the polyphenylene polyimide substrate used in thepresent process may or may not have a metal backing layer on its surfaceopposite to that on which the semiconductor material is to be deposited.Such a metal backing layer is useful in enhancing the mechanicalintegrity of the film during the transistor fabrication process, thusavoiding, for example, any tendency for the polyimide film to stretch orotherwise distort during handling, and thus reducing distortion of thesubstrate during formation of the transistors thereon. In addition, ametal backing layer can act as a light barrier to decrease any unwantedphoto-effects in the semiconductor material (for example, photogeneratedcurrent in an amorphous silicon film) caused by light incident on therear surface of the polyimide film. The metal backing layer need not becontinuous; this layer may have apertures extending through it to reduceits stiffness and thus give the metal-backed substrate more flexibility.If such apertures are to be provided, for obvious reasons it isdesirable that they be formed in a regular pattern, and accordingly someor all of the apertures may be used for mechanical registration of thesubstrate with apparatus used in the fabrication process. Indeed, insome cases, a patterned metal backing layer might be used as a shadowmask for exposure of photoresist in a patterning step during formationof the transistors on the substrate. Alternatively or in addition, itmay be advantageous to incorporate a dye into the polyimide itself torefuse or eliminate such undesirable photo-effects.

As in certain prior art processes, it may be desirable to deposit apassivating layer of silica, aluminum nitride, silicon nitride or othermaterial on the substrate prior to the deposition of the transistors onthis substrate. Typically, such a passivating layer will have athickness in the range of about 20 to about 100 nm. Passivation isuseful not only for increasing the surface resistance of the polyimidesurface, and thus for increasing electrical insulation between adjacentconductors, but also for increasing the dimensional stability of thesubstrate by preventing the substrate absorbing water during processing,and for the latter purpose it is desirable to place the passivatinglayer on both surfaces of the substrate. It is also desirable to heattreat (“bake”) the substrate to remove water from the substrate prior tothe deposition of the passivating layer; such baking will generally becarried out at a temperature of at least about 150° C. for a period ofat least about 1 minute, and preferably for about 3 minutes.

It may also be advantageous to post-bake the passivated substrate.According to a paper by Philips Research Laboratories entitled “AMLCDsand Electronics on Polymer Substrates” (Euro Display 1996), theshrinkage rate of a free-standing polyimide film can be reduced by twoorders of magnitude after 10 hours of heating at 275° C., and by 2.4orders of magnitude after 100 hours at the same temperature. In onespecific experiment described in this paper, after 100 baking at 275°C., a polyimide film (brand not specified) shrank at 3 ppm hr⁻¹.Accordingly, if such post-baking of the passivated substrate is desired,it should be carried out at a temperature of at least about 250° C. fora period of at least about 5 hours. It has not been determinedexperimentally whether these results apply the substrate in the form ofa tensioned roll, nor has it been determined experimentally whether thereduction in shrinkage still applies after the pre-baked substrate iscooled, unrolled, exposed to the processing necessary to formtransistors thereon, re-rolled under tension and reheated several dayslater, as is necessary for formation of transistors on the substrate ina roll-to-roll process. Alternatively, the substrate could be pre-bakedin a conveyor oven immediately before deposition of the layers requiredto form the transistor.

The surface electrical resistivity of polyphenylene polyimides is,however, so high (typically >10¹⁶ Ω) that in many cases it may bepossible to obtain adequate electrical insulation between adjacentconductors without such a passivating layer. If the passivating layer isomitted, it is still advantageous to bake the substrate beforedeposition of the semiconductor layer thereon in order to drive offwater and any other volatile materials absorbed on the polyimide, thusreducing swelling of the polyimide due to water absorption andincreasing the dimensional stability of the polyimide during theformation of transistors thereon. Such baking is desirably effected at atemperature greater than about 250° C. for a period of at least about 1hour. In one preferred embodiment described below, the substrate isheated to 350° C., close to its glass transition temperature, for aperiod of 4 hours.

The presently preferred embodiments of the invention described below usean inverted transistor design, in which the gate electrodes lie adjacentthe substrate. To form such inverted transistors, the first step (afterany passivation and/or pre-baking of the substrate in the ways alreadydescribed) is the deposition of a metal layer on the substrate. Thepreferred metal for this purpose is chromium. It is generally preferredto deposit the chromium or other metal as a continuous film, typicallyhaving a thickness in the range of about 50 to about 200 nm, andthereafter to pattern the metal film, typically by conventionalphotolithographic techniques, prior to the deposition of thesemiconductor material, to form the gate electrodes and the select linesof the transistor array to be formed. The next step in the process isnormally the deposition of a layer of dielectric material, for examplesilicon nitride; this deposition is conveniently effected by plasmaenhanced chemical vapor deposition. The semiconductor material,preferably amorphous silicon, is then deposited, again conveniently byplasma enhanced chemical vapor deposition. As discussed in theaforementioned copending application Ser. No. 09/565,413 and WO00/67327, the amorphous silicon layer (and the associated dielectriclayer) can, in an appropriate design, be left unpatterned so that theamorphous silicon layer extends continuously between pairs of adjacenttransistors. Next, a layer of n-type silicon is deposited over theamorphous silicon, again conveniently by plasma enhanced chemical vapordeposition. Finally, normally after a cleaning step to remove residuesfrom the chemical vapor deposition processes, a metal layer, for examplean aluminum layer, is deposited over the n-type silicon layer, thismetal layer conveniently being deposited by thermal evaporation. Themetal layer can then be patterned to form source and drain electrodes byconventional photolithographic techniques, and the patterned metal layerused as an etch mask for a reactive ion etch of the n-type siliconlayer; etching with a carbon tetrafluoride/oxygen mixture has been foundsatisfactory.

Preferred embodiments of the invention will now be described in moredetail, though by way of illustration only, with reference to theaccompanying drawing, which shows a schematic cross-section through asingle transistor formed on a polyphenylene polyimide substrate by theprocess of the present invention.

FIG. 1 of the accompanying drawings shows a single transistor of atransistor array (generally designated 10) formed on a polyphenylenepolyimide substrate 12. This substrate 12 is shown in the drawingprovided with a stainless steel metal backing layer 14 through whichextend regularly-spaced apertures 16, only one of which is visible inthe drawing. As already mentioned, the presence of the metal backinglayer 14 is optional in the process of the present invention, althoughsuch a metal backing layer does provide additional mechanical integrityto the substrate and may thus facilitate handling of the substrate,especially when the invention is to be carried out on roll-to-rollcoating apparatus.

On the upper surface of the substrate 12 (as shown in the drawing),there is deposited a passivating layer 18 formed of silica or siliconnitride. As previously mentioned, the presence of such a passivatinglayer 18 is optional, and in some cases the passivating layer 18 may beomitted, since the high surface resistivity of the polyphenylenepolyimide provides sufficient insulation between adjacent transistors.Care should, however, be taken in eliminating the passivating layer 18since if this layer is not present out-gassing from the polyimidesubstrate 12 may tend to cause delamination of various layers from thissubstrate.

On the upper surface of passivating layer 18, there are deposited anarray of spaced metal gate electrodes 20 (only one of which is seen inthe drawing), and above the electrodes 20 are deposited successively adielectric layer 22, formed of silicon nitride, and a layer 24 ofamorphous silicon. As discussed in the aforementioned copendingapplication Ser. No. 09/565,413 and WO 00/67327, the dielectric layer 22and the amorphous silicon layer 24 can be left unpatterned, and avoidingthe need to pattern these layers substantially reduces the cost of thetransistor array. Finally, the transistor array comprises a layer 26 ofn-type silicon and a metal electrode layer 28; both of these layers arepatterned using any conventional process to provide the source and drainelectrodes of the transistors.

An electrophoretic display can incorporate an array of transistors asdescribed above. Referring to FIG. 2 (which is identical, apart fromreference numerals, to FIG. 10 of the aforementioned copendingapplication Ser. No. 09/565,413), an electrophoretic display 100includes a substrate 101 supporting an electrode 102, a display medium106 provided next to the electrode 102, a plurality of pixel electrodes104 provided next to the display medium 106, and a plurality of discretetransistors provided next to and in electrical communication with thediscrete transistors. The gate electrodes 20, the gate dielectric layer22, the semiconductor layer 24 and the source electrodes of thetransistors are shown in this cross-section. The display medium 106 caninclude a plurality of microcapsules 124 dispersed in a binder.

A second preferred embodiment of the invention is generally similar tothat described above, but used a polyimide substrate without a metalbacking or passivating layer. In this second preferred embodiment, theaforementioned Upilex-50SS was first baked for 4 hours at 350° C. toremove water and any other solvents present. A layer of chrome 100 nm.thick was deposited upon the baked substrate by thermal evaporation andphotolithographically patterned to form the gate electrodes and selectlines of the final transistor array. Next, a 320 nm. layer of siliconnitride dielectric was deposited on the substrate by plasma enhancedchemical vapor deposition (PECVD) using a silane/ammonia mixture; duringthis deposition, the substrate reached its maximum processingtemperature of 350° C. A 160 nm layer of amorphous silicon semiconductormaterial was then deposited by PECVD from pure silane, followed bydeposition of a 40 nm layer of n-type amorphous silicon by PECVD from asilane/phosphine mixture.

Following these PECVD steps, a layer of aluminum was deposited on thesubstrate and patterned photolithographically to form the source anddrain electrodes of the transistor array. The substrate was thensubjected to a reactive ion etch using a carbon tetrafluoride/oxygenmixture to pattern the n-type silicon layer using the patterned aluminumlayer as an etch mask; for the reasons already explained, the amorphoussilicon and silicon nitride layers were not patterned during this step.Finally, a low resolution patterning step was used to pattern theamorphous silicon and silicon nitride layers to enable electricalcontact to be made with the select bond line sites.

The thin film transistor array thus fabricated can be used directly inthe manufacturer of an electrophoretic display, or other types ofdisplay, without further processing. For example, the thin filmtransistor array shown in the drawing could be incorporated into anelectrophoretic display by the process described in application Ser. No.09/461,463, filed Dec. 15, 1999; the entire disclosure of thisco-pending application is herein incorporated by reference. In somecases, it is desirable to provide a barrier layer covering the thin filmtransistors to protect the transistors against the effects of solventsor other materials which may tend to diffuse out of the electrophoreticdisplay.

As already mentioned, thin film transistor arrays produced by theprocess of the present invention are especially intended for use inelectrophoretic displays, especially encapsulated electrophoreticdisplays such as those described in U.S. Pat. Nos. 5,930,026; 5,961,804;6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851;6,130,773; 6,130,774; and 6,172,798, and in International ApplicationsPublication Nos. WO 97/04398; WO 98/03896; WO 98/19208; WO 98/41898; WO98/41899; WO 99/10769; WO 99/10768; WO 99/10767; WO 99/53373; WO99/56171; WO 99/59101; WO 99/47970; WO 00/03349; WO 00/03291; WO99/67678; WO 00/05704; WO 99/53371; WO 00/20921; WO 00/20922; WO00/20923; WO 00/26761; WO 00/36465; WO 00/38000; WO 00/38001; WO00/36560; WO 00/20922; WO 00/36666; WO 00/59625; WO 00/67110; WO00/67327 and WO 01/02899. The entire disclosures of all these patentsand published applications are herein incorporated by reference. In viewof the numerous different materials and manufacturing techniques whichcan be employed in such electrophoretic displays, the following SectionsA-E are given by way of general guidance.

A. Electrophoretic Particles

There is much flexibility in the choice of particles for use inelectrophoretic displays, as described above. For purposes of thisinvention, a particle is any component that is charged or capable ofacquiring a charge (i.e., has or is capable of acquiring electrophoreticmobility), and, in some cases, this mobility may be zero or close tozero (i.e., the particles will not move). The particles may be neatpigments, dyed (laked) pigments or pigment/polymer composites, or anyother component that is charged or capable of acquiring a charge.Typical considerations for the electrophoretic particle are its opticalproperties, electrical properties, and surface chemistry. The particlesmay be organic or inorganic compounds, and they may either absorb lightor scatter light. The particles for use in the invention may furtherinclude scattering pigments, absorbing pigments and luminescentparticles. The particles may be retroreflective, such as corner cubes,or they may be electroluminescent, such as zinc sulfide particles, whichemit light when excited by an AC field, or they may be photoluminescent.Zinc sulfide electroluminescent particles may be encapsulated with aninsulative coating to reduce electrical conduction. Finally, theparticles may be surface treated so as to improve charging orinteraction with a charging agent, or to improve dispersability.

One particle for use in electrophoretic displays of the invention istitania. The titania particles may be coated with a metal oxide, such asaluminum oxide or silicon oxide, for example. The titania particles mayhave one, two, or more layers of metal-oxide coating. For example, atitania particle for use in electrophoretic displays of the inventionmay have a coating of aluminum oxide and a coating of silicon oxide. Thecoatings may be added to the particle in any order.

The electrophoretic particle is usually a pigment, a polymer, a lakedpigment, or some combination of the above. A neat pigment can be anypigment, and, usually for a light colored particle, pigments such asrutile (titania), anatase (titania), barium sulfate, kaolin, or zincoxide are useful. Some typical particles have high refractive indices,high scattering coefficients, and low absorption coefficients. Otherparticles are absorptive, such as carbon black or colored pigments usedin paints and inks. The pigment should also be insoluble in thesuspending fluid. Yellow pigments such as diarylide yellow, Hansayellow, and benzidin yellow have also found use in similar displays. Anyother reflective material can be employed for a light colored particle,including non-pigment materials, such as metallic particles.

Useful neat pigments include, but are not limited to, PbCrO₄, Cyan blueGT 55-3295 (American Cyanamid Company, Wayne, N.J.), Cibacron Black BG(Ciba Company, Inc., Newport, Del.), Cibacron Turquoise Blue G (Ciba),Cibalon Black BGL (Ciba), Orasol Black BRG (Ciba), Orasol Black RBL(Ciba), Acetamine Black, CBS (E. I. du Pont de Nemours and Company,Inc., Wilmington, Del., hereinafter abbreviated “du Pont”), CroceinScarlet N Ex (du Pont) (27290), Fiber Black VF (du Pont) (30235), LuxolFast Black L (du Pont) (Solv. Black 17), Nirosine Base No. 424 (du Pont)(50415 B), Oil Black BG (du Pont) (Solv. Black 16), Rotalin Black RM (duPont), Sevron Brilliant Red 3 B (du Pont); Basic Black DSC (DyeSpecialties, Inc.), Hectolene Black (Dye Specialties, Inc.), AzosolBrilliant Blue B (GAF, Dyestuff and Chemical Division, Wayne, N.J.)(Solv. Blue 9), Azosol Brilliant Green BA (GAF) (Solv. Green 2), AzosolFast Brilliant Red B (GAF), Azosol Fast Orange RA Conc. (GAF) (Solv.Orange 20), Azosol Fast Yellow GRA Conc. (GAF) (13900 A), Basic BlackKMPA (GAF), Benzofix Black CW-CF (GAF) (35435), Cellitazol BNFV ExSoluble CF (GAF) (Disp. Black 9), Celliton Fast Blue AF Ex Conc (GAF)(Disp. Blue 9), Cyper Black IA (GAF) (Basic Black 3), Diamine Black CAPEx Conc (GAF) (30235), Diamond Black EAN Hi Con. CF (GAF) (15710),Diamond Black PBBA Ex (GAF) (16505); Direct Deep Black EA Ex CF (GAF)(30235), Hansa Yellow G (GAF) (11680); Indanthrene Black BBK Powd. (GAF)(59850), Indocarbon CLGS Conc. CF (GAF) (53295), Katigen Deep Black NNDHi Conc. CF (GAF) (15711), Rapidogen Black 3 G (GAF) (Azoic Black 4);Sulphone Cyanine Black BA-CF (GAF) (26370), Zambezi Black VD Ex Conc.(GAF) (30015); Rubanox Red CP-1495 (The Sherwin-Williams Company,Cleveland, Ohio) (15630); Raven 11 (Columbian Carbon Company, Atlanta,Ga.), (carbon black aggregates with a particle size of about 25 μm),Statex B-12 (Columbian Carbon Co.) (a furnace black of 33 μm averageparticle size), and chrome green.

Particles may also include laked, or dyed, pigments. Laked pigments areparticles that have a dye precipitated on them or which are stained.Lakes are metal salts of readily soluble anionic dyes. These are dyes ofazo, triphenylmethane or anthraquinone structure containing one or moresulphonic or carboxylic acid groupings. They are usually precipitated bya calcium, barium or aluminum salt onto a substrate. Typical examplesare peacock blue lake (CI Pigment Blue 24) and Persian orange (lake ofCI Acid Orange 7), Black M Toner (GAF) (a mixture of carbon black andblack dye precipitated on a lake).

A dark particle of the dyed type may be constructed from any lightabsorbing material, such as carbon black, or inorganic black materials.The dark material may also be selectively absorbing. For example, a darkgreen pigment may be used. Black particles may also be formed bystaining latices with metal oxides, such latex copolymers consisting ofany of butadiene, styrene, isoprene, methacrylic acid, methylmethacrylate, acrylonitrile, vinyl chloride, acrylic acid, sodiumstyrene sulfonate, vinyl acetate, chlorostyrene,dimethylaminopropylmethacrylamide, isocyanoethyl methacrylate andN-(isobutoxymethacrylamide), and optionally including conjugated dienecompounds such as diacrylate, triacrylate, dimethylacrylate andtrimethacrylate. Black particles may also be formed by a dispersionpolymerization technique.

In the systems containing pigments and polymers, the pigments andpolymers may form multiple domains within the electrophoretic particle,or be aggregates of smaller pigment/polymer combined particles.Alternatively, a central pigment core may be surrounded by a polymershell. The pigment, polymer, or both can contain a dye. The opticalpurpose of the particle may be to scatter light, absorb light, or both.Useful sizes may range from 1 nm up to about 100 μm, as long as theparticles are smaller than the bounding capsule. The density of theelectrophoretic particle may be substantially matched to that of thesuspending (i.e., electrophoretic) fluid. As defined herein, asuspending fluid has a density that is “substantially matched” to thedensity of the particle if the difference in their respective densitiesis between about zero and about two grams/milliliter (“g/ml”). Thisdifference is preferably between about zero and about 0.5 g/ml.

Useful polymers for the particles include, but are not limited to:polystyrene, polyethylene, polypropylene, phenolic resins, du Pont Elvaxresins (ethylene-vinyl acetate copolymers), polyesters, polyacrylates,polymethacrylates, ethylene acrylic acid or methacrylic acid copolymers(Nucrel Resins—du Pont, Primacor Resins—Dow Chemical), acryliccopolymers and terpolymers (Elvacite Resins—du Pont) and PMMA. Usefulmaterials for homopolymer/pigment phase separation in high shear meltinclude, but are not limited to, polyethylene, polypropylene,poly(methyl methacrylate), poly(isobutyl methacrylate), polystyrene,polybutadiene, polyisoprene, polyisobutylene, poly(lauryl methacrylate),poly(stearyl methacrylate), poly(isobornyl methacrylate), poly(t-butylmethacrylate), poly(ethyl methacrylate), poly(methyl acrylate),poly(ethyl acrylate), polyacrylonitrile, and copolymers of two or moreof these materials. Some useful pigment/polymer complexes that arecommercially available include, but are not limited to, Process MagentaPM 1776 (Magruder Color Company, Inc., Elizabeth, N.J.), Methyl VioletPMA VM6223 (Magruder Color Company, Inc., Elizabeth, N.J.), and NaphtholFGR RF6257 (Magruder Color Company, Inc., Elizabeth, N.J.).

The pigment-polymer composite may be formed by a physical process,(e.g., attrition or ball milling), a chemical process (e.g.,microencapsulation or dispersion polymerization), or any other processknown in the art of particle production. For example, the processes andmaterials for both the fabrication of liquid toner particles and thecharging of those particles may be relevant.

New and useful electrophoretic particles may still be discovered, but anumber of particles already known to those skilled in the art ofelectrophoretic displays and liquid toners can also prove useful. Ingeneral, the polymer requirements for liquid toners and encapsulatedelectrophoretic inks are similar, in that the pigment or dye must beeasily incorporated therein, either by a physical, chemical, orphysicochemical process, may aid in the colloidal stability, and maycontain charging sites or may be able to incorporate materials whichcontain charging sites. One general requirement from the liquid tonerindustry that is not shared by encapsulated electrophoretic inks is thatthe toner must be capable of “fixing” the image, i.e., heat fusingtogether to create a uniform film after the deposition of the tonerparticles.

Typical manufacturing techniques for particles may be drawn from theliquid toner and other arts and include ball milling, attrition, jetmilling, etc. The process will be illustrated for the case of apigmented polymeric particle. In such a case the pigment is compoundedin the polymer, usually in some kind of high shear mechanism such as ascrew extruder. The composite material is then (wet or dry) ground to astarting size of around 10 μm. It is then dispersed in a carrier liquid,for example ISOPAR® (Exxon, Houston, Tex.), optionally with some chargecontrol agent(s), and milled under high shear for several hours down toa final particle size and/or size distribution.

Another manufacturing technique for particles is to add the polymer,pigment, and suspending fluid to a media mill. The mill is started andsimultaneously heated to a temperature at which the polymer swellssubstantially with the solvent. This temperature is typically near 100°C. In this state, the pigment is easily encapsulated into the swollenpolymer. After a suitable time, typically a few hours, the mill isgradually cooled back to ambient temperature while stirring. The millingmay be continued for some time to achieve a small enough particle size,typically a few microns in diameter. The charging agents may be added atthis time. Optionally, more suspending fluid may be added.

Chemical processes such as dispersion polymerization, mini- ormicro-emulsion polymerization, suspension polymerization precipitation,phase separation, solvent evaporation, in situ polymerization, seededemulsion polymerization, or any process which falls under the generalcategory of microencapsulation may be used. A typical process of thistype is a phase separation process wherein a dissolved polymericmaterial is precipitated out of solution onto a dispersed pigmentsurface through solvent dilution, evaporation, or a thermal change.Other processes include chemical means for staining polymeric latices,for example with metal oxides or dyes.

B. Suspending Fluid

The suspending fluid containing the particles can be chosen based onproperties such as density, refractive index, and solubility. Apreferred suspending fluid has a low dielectric constant (about 2), highvolume resistivity (about 10¹⁵ ohm-cm), low viscosity (less than 5centistokes (“cst”)), low toxicity and environmental impact, low watersolubility (less than 10 parts per million (“ppm”)), high specificgravity (greater than 1.5), a high boiling point (greater than 90° C.),and a low refractive index (less than 1.2).

The choice of suspending fluid may be based on concerns of chemicalinertness, density matching to the electrophoretic particle, or chemicalcompatibility with both the electrophoretic particle and boundingcapsule. The viscosity of the fluid should be low when movement of theparticles is desired. The refractive index of the suspending fluid mayalso be substantially matched to that of the particles. As used herein,the refractive index of a suspending fluid “is substantially matched” tothat of a particle if the difference between their respective refractiveindices is between about zero and about 0.3, and is preferably betweenabout 0.05 and about 0.2.

Additionally, the fluid may be chosen to be a poor solvent for somepolymers, which is advantageous for use in the fabrication ofmicroparticles, because it increases the range of polymeric materialsuseful in fabricating particles of polymers and pigments. Organicsolvents, such as halogenated organic solvents, saturated linear orbranched hydrocarbons, silicone oils, and low molecular weighthalogen-containing polymers are some useful suspending fluids. Thesuspending fluid may comprise a single fluid. The fluid will, however,often be a blend of more than one fluid in order to tune its chemicaland physical properties. Furthermore, the fluid may contain surfacemodifiers to modify the surface energy or charge of the electrophoreticparticle or bounding capsule. Reactants or solvents for themicroencapsulation process (oil soluble monomers, for example) can alsobe contained in the suspending fluid. Charge control agents can also beadded to the suspending fluid.

Useful organic solvents include, but are not limited to, epoxides, suchas decane epoxide and dodecane epoxide; vinyl ethers, such as cyclohexylvinyl ether and Decave® (International Flavors & Fragrances, Inc., NewYork, N.Y.); and aromatic hydrocarbons, such as toluene and naphthalene.Useful halogenated organic solvents include, but are not limited to,tetrafluorodibromoethylene, tetrachloroethylene,trifluorochloroethylene, 1,2,4-trichlorobenzene and carbontetrachloride. These materials have high densities. Useful hydrocarbonsinclude, but are not limited to, dodecane, tetradecane, the aliphatichydrocarbons in the Isopar® series (Exxon, Houston, Tex.), Norpar® (aseries of normal paraffinic liquids), Shell-Sol® (Shell, Houston, Tex.),and Sol-Trol® ((Shell), naphtha, and other petroleum solvents. Thesematerials usually have low densities. Useful examples of silicone oilsinclude, but are not limited to, octamethyl cyclosiloxane and highermolecular weight cyclic siloxanes, poly(methyl phenyl siloxane),hexamethyldisiloxane, and polydimethylsiloxane. These materials usuallyhave low densities. Useful low molecular weight halogen-containingpolymers include, but are not limited to, poly(chlorotrifluoroethylene)polymer (Halogenated Hydrocarbon Inc., River Edge, N.J.), Galden® (aperfluorinated ether from Ausimont, Morristown, N.J.), or Krytox® fromdu Pont (Wilmington, Del.). In a preferred embodiment, the suspendingfluid is a poly(chlorotrifluoroethylene) polymer. In a particularlypreferred embodiment, this polymer has a degree of polymerization fromabout 2 to about 10. Many of the above materials are available in arange of viscosities, densities, and boiling points.

The fluid must be capable of being formed into small droplets prior to acapsule being formed. Processes for forming small droplets includeflow-through jets, membranes, nozzles, or orifices, as well asshear-based emulsifying schemes. The formation of small drops may beassisted by electrical or sonic fields. Surfactants and polymers can beused to aid in the stabilization and emulsification of the droplets inthe case of an emulsion type encapsulation. One surfactant for use indisplays of the invention is sodium dodecylsulfate.

It can be advantageous in some displays for the suspending fluid tocontain an optically absorbing dye. This dye must be soluble in thefluid, but will generally be insoluble in the other components of thecapsule. There is much flexibility in the choice of dye material. Thedye can be a pure compound, or blends of dyes to achieve a particularcolor, including black. The dyes can be fluorescent, which would producea display in which the fluorescence properties depend on the position ofthe particles. The dyes can be photoactive, changing to another color orbecoming colorless upon irradiation with either visible or ultravioletlight, providing another means for obtaining an optical response. Dyescould also be polymerizable by, for example, thermal, photochemical orchemical diffusion processes, forming a solid absorbing polymer insidethe bounding shell.

There are many dyes that can be used in encapsulated electrophoreticdisplays. Properties important here include light fastness, solubilityin the suspending liquid, color, and cost. These dyes are generallychosen from the classes of azo, anthraquinone, and triphenylmethane typedyes and may be chemically modified so as to increase their solubilityin the oil phase and reduce their adsorption by the particle surface.

A number of dyes already known to those skilled in the art ofelectrophoretic displays will prove useful. Useful azo dyes include, butare not limited to: the Oil Red dyes, and the Sudan Red and Sudan Blackseries of dyes. Useful anthraquinone dyes include, but are not limitedto: the Oil Blue dyes, and the Macrolex Blue series of dyes. Usefultriphenylmethane dyes include, but are not limited to, Michler's hydrol,Malachite Green, Crystal Violet, and Auramine O.

The ratio of particles to suspending fluid to suspending fluid may varyover a wide range depending upon, inter alia, the density and opacity ofthe particles, the desired switching speed of the display and the degreeof bistability desired. Typically, the particles will comprise fromabout 0.5 percent to about 20 percent by weight of the internal phase.However, in some dual particle systems, it may be advantageous to usesubstantially higher particle loadings in order to enhance thebistability of the images produced. Dual particle electrophoretic mediain which the two types of particles carry charges of opposite polarityflocculate naturally because of the electrostatic attraction between theoppositely-charged particles. At high particles loadings, with theparticles constituting around 50 to 70 weight percent of the internalphase, the resultant floc structure essentially fills the volume of theinternal phase and holds the particles close to their addressed state(i.e., close to the positions which they occupy after an electric fieldhas been applied to the medium for a period sufficient to drive thedisplay to one of its two extreme optical states), thus enhancing thebistability of the display. The density, strength and rate offlocculation are readily controlled by adjusting particle charge, sizeand steric barrier thickness and composition. This method of increasingby increasing particle loading has the advantage that no extraneousmaterial is added to the internal phase, and that the floc structurewill stabilize not only the two extreme optical states but also theintermediate “gray” states. Also, this method reduces the temperaturesensitivity of the stable states and reduces sticking of the particlesto the capsule walls. The Bingham viscosity of the internal phaseremains low, and even small floc volumes will aid in maintaining imagebistability. Finally, the floc structure is easily broken by a shortalternating current pulse, which can readily be applied before thedirect current pulse used to alter the optical state of the display.

C. Charge Control Agents and Particle Stabilizers

Charge control agents are used to provide good electrophoretic mobilityto the electrophoretic particles. Stabilizers are used to preventagglomeration of the electrophoretic particles, as well as prevent theelectrophoretic particles from irreversibly depositing onto the capsulewall. Either component can be constructed from materials across a widerange of molecular weights (low molecular weight, oligomeric, orpolymeric), and may be a single pure compound or a mixture. The chargecontrol agent used to modify and/or stabilize the particle surfacecharge is applied as generally known in the arts of liquid toners,electrophoretic displays, non-aqueous paint dispersions, and engine-oiladditives. In all of these arts, charging species may be added tonon-aqueous media in order to increase electrophoretic mobility orincrease electrostatic stabilization. The materials can improve stericstabilization as well. Different theories of charging are postulated,including selective ion adsorption, proton transfer, and contactelectrification.

An optional charge control agent or charge director may be used. Theseconstituents typically consist of low molecular weight surfactants,polymeric agents, or blends of one or more components and serve tostabilize or otherwise modify the sign and/or magnitude of the charge onthe electrophoretic particles. The charging properties of the pigmentitself may be accounted for by taking into account the acidic or basicsurface properties of the pigment, or the charging sites may take placeon the carrier resin surface (if present), or a combination of the two.Additional pigment properties which may be relevant are the particlesize distribution, the chemical composition, and the lightfastness.

Charge adjuvants may also be added. These materials increase theeffectiveness of the charge control agents or charge directors. Thecharge adjuvant may be a polyhydroxy compound or an aminoalcoholcompound, and is preferably soluble in the suspending fluid in an amountof at least 2% by weight. Examples of polyhydroxy compounds whichcontain at least two hydroxyl groups include, but are not limited to,ethylene glycol, 2,4,7,9-tetramethyldecyn-4,7-diol, poly(propyleneglycol), pentaethylene glycol, tripropylene glycol, triethylene glycol,glycerol, pentaerythritol, glycerol tris(12-hydroxystearate), propyleneglycerol monohydroxy-stearate, and ethylene glycol monohydroxystearate.Examples of aminoalcohol compounds which contain at least one alcoholfunction and one amine function in the same molecule include, but arenot limited to, triisopropanolamine, triethanolamine, ethanolamine,3-amino-1-propanol, o-aminophenol, 5-amino-1-pentanol, andtetrakis(2-hydroxyethyl)ethylenediamine. The charge adjuvant ispreferably present in the suspending fluid in an amount of about 1 toabout 100 milligrams per gram (“mg/g”) of the particle mass, and morepreferably about 50 to about 200 mg/g.

The surface of the particle may also be chemically modified to aiddispersion, to improve surface charge, and to improve the stability ofthe dispersion, for example. Surface modifiers include organicsiloxanes, organohalogen silanes and other functional silane couplingagents (Dow Corning® Z-6070, Z-6124, and 3 additive, Midland, Mich.);organic titanates and zirconates (Tyzor® TOT, TBT, and TE Series, duPont); hydrophobing agents, such as long chain (C₁₂ to C₅₀) alkyl andalkyl benzene sulphonic acids, fatty amines or diamines and their saltsor quaternary derivatives; and amphipathic polymers which can becovalently bonded to the particle surface.

In general, it is believed that charging results as an acid-basereaction between some moiety present in the continuous phase and theparticle surface. Thus useful materials are those which are capable ofparticipating in such a reaction, or any other charging reaction asknown in the art.

Different non-limiting classes of charge control agents which are usefulinclude organic sulfates or sulfonates, metal soaps, block or combcopolymers, organic amides, organic zwitterions, and organic phosphatesand phosphonates. Useful organic sulfates and sulfonates include, butare not limited to, sodium bis(2-ethylhexyl) sulfosuccinate, calciumdodecylbenzenesulfonate, calcium petroleum sulfonate, neutral or basicbarium dinonylnaphthalene sulfonate, neutral or basic calciumdinonylnaphthalene sulfonate, dodecylbenzenesulfonic acid sodium salt,and ammonium lauryl sulfate. Useful metal soaps include, but are notlimited to, basic or neutral barium petronate, calcium petronate, Co—,Ca—, Cu—, Mn—, Ni—, Zn—, and Fe— salts of naphthenic acid, Ba—, Al—,Zn—, Cu—, Pb—, and Fe— salts of stearic acid, divalent and trivalentmetal carboxylates, such as aluminum tristearate, aluminum octanoate,lithium heptanoate, iron stearate, iron distearate, barium stearate,chromium stearate, magnesium octanoate, calcium stearate, ironnaphthenate, zinc naphthenate, Mn— and Zn— heptanoate, and Ba—, Al—,Co—, Mn—, and Zn— octanoate. Useful block or comb copolymers include,but are not limited to, AB diblock copolymers of (A) polymers of2-(N,N-dimethylamino)ethyl methacrylate quaternized with methylp-toluenesulfonate and (B) poly(2-ethylhexyl methacrylate), and combgraft copolymers with oil soluble tails of poly(12-hydroxystearic acid)and having a molecular weight of about 1800, pendant on an oil-solubleanchor group of poly(methyl methacrylate-methacrylic acid). Usefulorganic amides include, but are not limited to, polyisobutylenesuccinimides such as OLOA 1200, and N-vinylpyrrolidone polymers. Usefulorganic zwitterions include, but are not limited to, lecithin. Usefulorganic phosphates and phosphonates include, but are not limited to, thesodium salts of phosphated mono- and di-glycerides with saturated andunsaturated acid substituents.

Particle dispersion stabilizers may be added to prevent particleflocculation or attachment to the capsule walls. For the typical highresistivity liquids used as suspending fluids in electrophoreticdisplays, non-aqueous surfactants may be used. These include, but arenot limited to, glycol ethers, acetylenic glycols, alkanolamides,sorbitol derivatives, alkyl amines, quaternary amines, imidazolines,dialkyl oxides, and sulfosuccinates.

D. Encapsulation

Encapsulation of the internal phase may be accomplished in a number ofdifferent ways. Numerous suitable procedures for microencapsulation aredetailed in both Microencapsulation, Processes and Applications, (I. E.Vandegaer, ed.), Plenum Press, New York, N.Y. (1974) and Gutcho,Microcapsules and Microencapsulation Techniques, Noyes Data Corp., ParkRidge, N.J. (1976). The processes fall into several general categories,all of which can be applied to the present invention: interfacialpolymerization, in situ polymerization, physical processes, such ascoextrusion and other phase separation processes, in-liquid curing, andsimple/complex coacervation.

Numerous materials and processes should prove useful in formulatingdisplays of the present invention. Useful materials for simplecoacervation processes to form the capsule include, but are not limitedto, gelatin, poly(vinyl alcohol), poly(vinyl acetate), and cellulosicderivatives, such as, for example, carboxymethylcellulose. Usefulmaterials for complex coacervation processes include, but are notlimited to, gelatin, acacia, carageenan, carboxymethylcellulose,hydrolyzed styrene anhydride copolymers, agar, alginate, casein,albumin, methyl vinyl ether co-maleic anhydride, and cellulosephthalate. Useful materials for phase separation processes include, butare not limited to, polystyrene, poly(methyl methacrylate) (PMMA),poly(ethyl methacrylate), poly(butyl methacrylate), ethyl cellulose,poly(vinylpyridine), and polyacrylonitrile. Useful materials for in situpolymerization processes include, but are not limited to,polyhydroxyamides, with aldehydes, melamine, or urea and formaldehyde;water-soluble oligomers of the condensate of melamine, or urea andformaldehyde; and vinyl monomers, such as, for example, styrene, methylmethacrylate (MMA) and acrylonitrile. Finally, useful materials forinterfacial polymerization processes include, but are not limited to,diacyl chlorides, such as, for example, sebacoyl, adipoyl, and di- orpoly-amines or alcohols, and isocyanates. Useful emulsion polymerizationmaterials may include, but are not limited to, styrene, vinyl acetate,acrylic acid, butyl acrylate, t-butyl acrylate, methyl methacrylate, andbutyl methacrylate.

Capsules produced may be dispersed into a curable carrier, resulting inan ink which may be printed or coated on large and arbitrarily shaped orcurved surfaces using conventional printing and coating techniques.

In the context of the present invention, one skilled in the art willselect an encapsulation procedure and wall material based on the desiredcapsule properties. These properties include the distribution of capsuleradii; electrical, mechanical, diffusion, and optical properties of thecapsule wall; and chemical compatibility with the internal phase of thecapsule.

The capsule wall generally has a high electrical resistivity. Althoughit is possible to use walls with relatively low resistivities, this maylimit performance in requiring relatively higher addressing voltages.The capsule wall should also be mechanically strong (although if thefinished capsule powder is to be dispersed in a curable polymeric binderfor coating, mechanical strength is not as critical). The capsule wallshould generally not be porous. If, however, it is desired to use anencapsulation procedure that produces porous capsules, these can beovercoated in a post-processing step (i.e., a second encapsulation).Moreover, if the capsules are to be dispersed in a curable binder, thebinder will serve to close the pores. The capsule walls should beoptically clear. The wall material may, however, be chosen to match therefractive index of the internal phase of the capsule (i.e., thesuspending fluid) or a binder in which the capsules are to be dispersed.For some applications (e.g., interposition between two fixedelectrodes), monodispersed capsule radii are desirable.

An encapsulation technique that is suited to the present inventioninvolves a polymerization between urea and formaldehyde in an aqueousphase of an oil/water emulsion in the presence of a negatively charged,carboxyl-substituted, linear hydrocarbon polyelectrolyte material. Theresulting capsule wall is a urea/formaldehyde copolymer, whichdiscretely encloses the internal phase. The capsule is clear,mechanically strong, and has good resistivity properties.

The related technique of in situ polymerization utilizes an oil/wateremulsion, which is formed by dispersing the electrophoretic fluid (i.e.,the dielectric liquid containing a suspension of the pigment particles)in an aqueous environment. The monomers polymerize to form a polymerwith higher affinity for the internal phase than for the aqueous phase,thus condensing around the emulsified oily droplets. In one in situpolymerization process, urea and formaldehyde condense in the presenceof poly(acrylic acid) (see, e.g., U.S. Pat. No. 4,001,140). In otherprocesses, described in U.S. Pat. No. 4,273,672, any of a variety ofcross-linking agents borne in aqueous solution is deposited aroundmicroscopic oil droplets. Such cross-linking agents include aldehydes,especially formaldehyde, glyoxal, or glutaraldehyde; alum; zirconiumsalts; and polyisocyanates.

The coacervation approach also utilizes an oil/water emulsion. One ormore colloids are coacervated (i.e., agglomerated) out of the aqueousphase and deposited as shells around the oily droplets through controlof temperature, pH and/or relative concentrations, thereby creating themicrocapsule. Materials suitable for coacervation include gelatins andgum arabic. See, e.g., U.S. Pat. No. 2,800,457.

The interfacial polymerization approach relies on the presence of anoil-soluble monomer in the electrophoretic composition, which once againis present as an emulsion in an aqueous phase. The monomers in theminute hydrophobic droplets react with a monomer introduced into theaqueous phase, polymerizing at the interface between the droplets andthe surrounding aqueous medium and forming shells around the droplets.Although the resulting walls are relatively thin and may be permeable,this process does not require the elevated temperatures characteristicof some other processes, and therefore affords greater flexibility interms of choosing the dielectric liquid.

Coating aids can be used to improve the uniformity and quality of thecoated or printed electrophoretic ink material. Wetting agents aretypically added to adjust the interfacial tension at thecoating/substrate interface and to adjust the liquid/air surfacetension. Wetting agents include, but are not limited to, anionic andcationic surfactants, and nonionic species, such as silicone orfluoropolymer-based materials. Dispersing agents may be used to modifythe interfacial tension between the capsules and binder, providingcontrol over flocculation and particle settling.

Surface tension modifiers can be added to adjust the air/ink interfacialtension. Polysiloxanes are typically used in such an application toimprove surface leveling while minimizing other defects within thecoating. Surface tension modifiers include, but are not limited to,fluorinated surfactants, such as, for example, the Zonyl® series from duPont, the Fluorad® series from 3M (St. Paul, Minn.), and the fluoroalkylseries from Autochem (Glen Rock, N.J.); siloxanes, such as, for example,Silwet® from Union Carbide (Danbury, Conn.); and polyethoxy andpolypropoxy alcohols. Antifoams, such as silicone and silicone-freepolymeric materials, may be added to enhance the movement of air fromwithin the ink to the surface and to facilitate the rupture of bubblesat the coating surface. Other useful antifoams include, but are notlimited to, glyceryl esters, polyhydric alcohols, compounded antifoams,such as oil solutions of alkylbenzenes, natural fats, fatty acids, andmetallic soaps, and silicone antifoaming agents made from thecombination of dimethyl siloxane polymers and silica. Stabilizers suchas UV-absorbers and antioxidants may also be added to improve thelifetime of the ink.

E. Binder Material

The binder typically is used as an adhesive medium that supports andprotects the capsules, as well as binds the electrode materials to thecapsule dispersion. A binder can be non-conducting, semiconductive, orconductive. Binders are available in many forms and chemical types.Among these are water-soluble polymers, water-borne polymers,oil-soluble polymers, thermoset and thermoplastic polymers, andradiation-cured polymers.

Among the water-soluble polymers are the various polysaccharides, thepolyvinyl alcohols, N-methylpyrrolidone, N-vinylpyrrolidone, the variousCarbowax® species (Union Carbide, Danbury, Conn.), andpoly(2-hydroxyethyl acrylate).

The water-dispersed or water-borne systems are generally latexcompositions, typified by the Neorez® and Neocryl® resins (ZenecaResins, Wilmington, Mass.), Acrysol® (Rohm and Haas, Philadelphia, Pa.),Bayhydrol® (Bayer, Pittsburgh, Pa.), and the Cytec Industries (WestPaterson, N.J.) HP line. These are generally latices of polyurethanes,occasionally compounded with one or more of the acrylics, polyesters,polycarbonates or silicones, each lending the final cured resin in aspecific set of properties defined by glass transition temperature,degree of “tack,” softness, clarity, flexibility, water permeability andsolvent resistance, elongation modulus and tensile strength,thermoplastic flow, and solids level. Some water-borne systems can bemixed with reactive monomers and catalyzed to form more complex resins.Some can be further cross-linked by the use of a cross-linking reagent,such as an aziridine, for example, which reacts with carboxyl groups.

A typical application of a water-borne resin and aqueous capsulesfollows. A volume of particles is centrifuged at low speed to separateexcess water. After a given centrifugation process, for example 10minutes at 60×gravity (“g”), the capsules 180 are found at the bottom ofthe centrifuge tube 182, while the water portion 184 is at the top. Thewater portion is carefully removed (by decanting or pipetting). The massof the remaining capsules is measured, and a mass of resin is added suchthat the mass of resin is, for example, between one eighth and one tenthof the weight of the capsules. This mixture is gently mixed on anoscillating mixer for approximately one half hour. After about one halfhour, the mixture is ready to be coated onto the appropriate substrate.

The thermoset systems are exemplified by the family of epoxies. Thesebinary systems can vary greatly in viscosity, and the reactivity of thepair determines the “pot life” of the mixture. If the pot life is longenough to allow a coating operation, capsules may be coated in anordered arrangement in a coating process prior to the resin curing andhardening.

Thermoplastic polymers, which are often polyesters, are molten at hightemperatures. A typical application of this type of product is hot-meltglue. A dispersion of heat-resistant capsules could be coated in such amedium. The solidification process begins during cooling, and the finalhardness, clarity and flexibility are affected by the branching andmolecular weight of the polymer.

Oil or solvent-soluble polymers are often similar in composition to thewater-borne system, with the obvious exception of the water itself. Thelatitude in formulation for solvent systems is enormous, limited only bysolvent choices and polymer solubility. Of considerable concern insolvent-based systems is the viability of the capsule itself; theintegrity of the capsule wall cannot be compromised in any way by thesolvent.

Radiation cure resins are generally found among the solvent-basedsystems. Capsules may be dispersed in such a medium and coated, and theresin may then be cured by a timed exposure to a threshold level ofultraviolet radiation, either long or short wavelength. As in all casesof curing polymer resins, final properties are determined by thebranching and molecular weights of the monomers, oligomers andcross-linkers.

A number of “water-reducible” monomers and oligomers are, however,marketed. In the strictest sense, they are not water soluble, but wateris an acceptable diluent at low concentrations and can be dispersedrelatively easily in the mixture. Under these circumstances, water isused to reduce the viscosity (initially from thousands to hundreds ofthousands centipoise). Water-based capsules, such as those made from aprotein or polysaccharide material, for example, could be dispersed insuch a medium and coated, provided the viscosity could be sufficientlylowered. Curing in such systems is generally by ultraviolet radiation.

Like other encapsulated electrophoretic displays, the encapsulatedelectrophoretic displays of the present invention provide flexible,reflective displays that can be manufactured easily and consume littlepower (or no power in the case of bistable displays in certain states).Such displays, therefore, can be incorporated into a variety ofapplications and can take on many forms. Once the electric field isremoved, the electrophoretic particles can be generally stable.Additionally, providing a subsequent electric charge can alter a priorconfiguration of particles. Such displays may include, for example, aplurality of anisotropic particles and a plurality of second particlesin a suspending fluid. Application of a first electric field may causethe anisotropic particles to assume a specific orientation and presentan optical property. Application of a second electric field may thencause the plurality of second particles to translate, therebydisorienting the anisotropic particles and disturbing the opticalproperty. Alternatively, the orientation of the anisotropic particlesmay allow easier translation of the plurality of second particles.Alternatively or in addition, the particles may have a refractive indexthat substantially matches the refractive index of the suspending fluid.

An encapsulated electrophoretic display may take many forms. Thecapsules of such a display may be of any size or shape. The capsulesmay, for example, be spherical and may have diameters in the millimeterrange or the micron range, but are preferably from about ten to about afew hundred microns. The particles within the capsules of such a displaymay be colored, luminescent, light-absorbing or transparent, forexample.

Numerous changes and modifications can be made in the preferredembodiments of the present process already described without departingfrom the spirit and skill of the invention. For example, the presentinvention is not restricted to the fabrication of bottom gatetransistors such as that shown in the accompanying drawings, the courtalso be used in the fabrication of top gate transistors, in which thesource and drain electrodes are first fabricated on the substrate (withor without a passivating layer), then an amorphous silicon layer and adielectric layer are formed on top of the electrodes, and finally thegate electrodes are formed as the top layer of the structure.Accordingly, the foregoing description is to be construed in anillustrative and not in a limitative sense.

From the foregoing, it will be seen that the process of the presentinvention provides a process for forming transistors on a flexiblesubstrate which permits the use of higher processing temperatures thanprior art processes, and which can thus produce semiconductor layers ofhigher quality than prior art processes. The substrate used in thepresent process has a coefficient of thermal expansion which closelymatches that of most semiconductor layers, so reducing the risk ofcracking and/or delamination of the semiconductor layer due todifferences in thermal expansion between this layer and the substrate.The present invention provides a process which is well-adapted toroll-to-roll operation, and thus the present process is very suitablefor the fabrication of large area transistor arrays on flexiblesubstrates.

1. An electrophoretic display comprising, in order: a metal layer; apolyimide substrate; at least one transistor on the polyimide substrate;and an electrophoretic medium having a plurality of electrophoreticparticles dispersed in a suspending fluid and capable of movingtherethrough on application of an electric field to the medium.
 2. Anelectrophoretic display according to claim 1 wherein the metal layercomprises stainless steel.
 3. An electrophoretic display according toclaim 1 wherein the electrophoretic particles and the suspending fluidare encapsulated within a plurality of capsules.
 4. An electrophoreticdisplay comprising, in order: a metal layer; a polyimide substrate; atleast one transistor on the polyimide substrate; and an electrophoreticmedium having a plurality of electrophoretic particles dispersed in asuspending fluid and capable of moving therethrough on application of anelectric field to the medium, wherein the polyimide is a polyphenylenepolyimide.
 5. An electrophoretic display according to claim 4 whereinthe polyphenylene polyimide is a derivative ofbiphenyl-3,3′,4,4′-tetracarboxylic acid.
 6. An electrophoretic displayaccording to claim 5 wherein the polyimide is a derivative ofbiphenyl-3,3′,4,4′-tetracarboxylic acid and an a,ω-alkanediamine.
 7. Anelectrophoretic display comprising, in order: a metal layer; a polyimidesubstrate; at least one transistor on the polyimide substrate; and anelectrophoretic medium having a plurality of electrophoretic particlesdispersed in a suspending fluid and capable of moving therethrough onapplication of an electric field to the medium, the display furthercomprising a passivating layer between the polyimide substrate and theat least one transistor.
 8. An electrophoretic display according toclaim 7 wherein the passivating layer comprises silicon dioxide oraluminum nitride.
 9. An electrophoretic display according to claim 7wherein the passivating layer has a thickness in the range of about 20to about 100 nm.
 10. An electrophoretic display according to claim 7wherein the passivating layer is deposited on both surfaces of thesubstrate.
 11. An electrophoretic display comprising, in order: a metallayer having walls defining apertures extending through the metal layer;a polyimide substrate; at least one transistor on the polyimidesubstrate; and an electrophoretic medium having a plurality ofelectrophoretic particles dispersed in a suspending fluid and capable ofmoving therethrough on application of an electric field to the medium.12. An electrophoretic display comprising, in order: a metal layer; apolyimide substrate; at least one transistor on the polyimide substrate;and an electrophoretic medium having a plurality of electrophoreticparticles dispersed in a suspending fluid and capable of movingtherethrough on application of an electric field to the medium, whereinthe transistor comprises at least one amorphous silicon layer.
 13. Anelectrophoretic display according to claim 12 wherein the polyimidesubstrate bears at least two transistors, each of the transistorscomprising at least one amorphous silicon layer and the silicon layerextending continuously between the two transistors.
 14. Anelectrophoretic display according to claim 12 wherein the transistorfurther comprises at least one n-type silicon layer in contact with theamorphous silicon layer.